1. Field of the Invention
The present invention relates to a helicopter blade aerofoil.
2. Description of the Related Art
FIGS. 6A and 6B are illustrative views for showing aerodynamic characteristics of a rotor of a helicopter in forward flight. As shown in FIG. 6A, when a helicopter 1 with a rotor having a radius R which is rotating at a rotational angular velocity .OMEGA. advances at a ground velocity V, the airspeeds of an advancing blade where the rotational speed of the rotor is added to the ground velocity V and a retreating blade where the ground velocity V is subtracted from the rotational speed of the rotor are significantly different.
In particular, at a position of an azimuth angle .PSI. (angle measured counterclockwise from the rearward direction of the helicopter 1) of 90.degree., the airspeed of the advancing blade reaches a maximum and the airspeed of a tip of the advancing blade becomes .OMEGA..times.R+V. At a position of .PSI.=270.degree., on the other hand, the airspeed of the retreating blade reaches a minimum and the airspeed of a tip of the retreating blade becomes .OMEGA..times.R-V. The airspeed of an intermediate portion of the blade takes a value obtained by proportional distribution of .OMEGA..times.R+V and .OMEGA..times.R-V. For example, when .OMEGA..times.R=795 km/h and V=278 km/h are assumed, the airspeed at a position of about 35% from the root end of the retreating blade becomes zero, as shown in FIG. 6A.
Since the airspeeds of the blades thus vary greatly while the blades make one revolution, various phenomena take place. On an advancing blade, drag coefficient Cd increases rapidly as the airspeeds approach the speed of sound. When the airspeeds are given in terms of Mach number M, drag divergence Mach number Mdd is defined as Mach number of a time when increment .DELTA. Cd of drag coefficient Cd divided by increment .DELTA. M of Mach number (.DELTA. Cd/.DELTA. M) becomes 0.1. Drag divergence Mach number Mdd depends on a blade aerofoil section, and it is said that the greater the value, the better the blade becomes because a higher airspeed of the blade can be achieved. It is common to set the airspeed of the tip of the advancing blade to around Mach 0.85.
On a retreating blade, on the other hand, since the airspeed thereof is significantly lowered, angle of attack .alpha. of the retreating blade must be greater in order to produce a lift similar to that of the advancing blade. For this purpose, it is common to carry out pitch control wherein a pitch angle of the retreating blade is controlled in accordance to azimuth angle .PSI.. While the pitch angle of the blade is controlled as a sinewave function which has a minimum amplitude at .PSI.=90.degree. and a maximum amplitude at .PSI.=270.degree., angle of attack .alpha. of the blade in this case varies in the direction of span as shown in FIG. 6B due to flapping of the blade itself. For example, when the blade is at the position of .PSI.=90.degree., the angle of attack a becomes about 0.degree. at the root end and about 4.degree. at the tip end. When .PSI.=270.degree., the angle of attack .alpha. of the blade becomes about 0.degree. at the root end and about 16.degree. to 18.degree. at the tip end, thus exceeding the stall angle of attack.
Characteristics used for evaluating a retreating blade include maximum lift coefficient Clmax and stall angle of attack, the maximum lift coefficient Clmax is defined as the maximum value of lift coefficient when the angle of attack .alpha. of a blade having a particular aerofoil section is gradually increased and reached the stall angle of attack. The blade is said to be better when the values of maximum lift coefficient Clmax and stall angle of attack are greater.
FIG. 7 is a graph showing an operating environment of helicopter rotor blades. The advancing blade at .PSI.=90.degree. has a Mach number near the drag divergence Mach number Mdd and a lift coefficient Cl of about zero. The blade at .PSI.=0.degree. and 180.degree. is in a hovering state which is independent of the ground velocity V, while Mach number M is about 0.6 and lift coefficient Cl is about 0.6. The retreating blade at .PSI.=270.degree. has a Mach number of 0.3 to 0.5 and a lift coefficient Cl near the maximum lift coefficient Clmax. As the blade makes a full revolution, Mach number and lift coefficient vary greatly by going around these states described above.
Hence a helicopter blade aerofoil is required 1) to have a large value of drag divergence Mach number Mdd, and 2) to have a large value of maximum lift coefficient Clmax, while a better flight performance of a helicopter is achieved when these values are greater.
The above description applies to an aerofoil of the tip side of a blade where the air speed is high. On the other hand, at the root end side of the blade where the air speed is not so high, greater emphasis is placed on maximum lift coefficient Clmax than on drag divergence Mach number Mdd. Further, a rotor blade of high efficiency with respect to lift or higher lift-drag ratio (lift/drag) is desired.
FIG. 8 is a graph explaining the lift-drag ratio (L/D) of a helicopter blade. This graph, plotting angles of attack(degrees) along the axis of abscissa, and the lift coefficient and drag coefficient along the axis of ordinate, shows changes of lift coefficient Cl and drag coefficient Cd versus angle of attack, when Mach number M is 0.6.
As a general aerodynamic phenomenon of aerofoil, lift and drag coefficients increase as the angle of attack increases. In a helicopter blade aerofoil, the higher the lift-drag ratio, the better the torque efficiency. Since the pitch angle of a blade changes periodically during rotation of the rotor, as described above, the lift-drag ratio calculated in such a manner is evaluated that a value of drag coefficient Cd in the case of Machnumber M=0.6 and lift coefficient Cl=0.6 is determined and Cl is divided by Cd.
Thus, at the tip side of the blade higher drag divergence Mach number Mdd and higher maximum lift coefficient Clmax are preferred, and at the root end side of the blade higher maximum lift coefficient Clmax is preferred, while higher lift-drag ratio (L/D) is preferred for the entire blade.
There are known as prior arts related to helicopter blade aerofoils, Japanese Unexamined Patent Publications JP-A 50-102099(1975), JP-A 59-134096(1984), and Japanese Examined Patent Publication JP-B2 62-34600(1987), for example.
Recently such attempts have been proposed that helicopters take off and land regularly by using roof top heliport of buildings or operate public area heliports, and for which it is required to minimize the noise of helicopters in flight.
FIG. 9 is a graph showing frequency spectra of noises generated by a helicopter. The noises of the helicopter are classified into several categories on the basis of origins of the noises, while harmonic components of the main rotor rotation frequency are distributed in a range from 10 to 100 Hz, harmonic components of the tail rotor rotation frequency are distributed in a range from 60 to 300 Hz, and broadband noise of the main rotor is distributed from 60 to 300 Hz. When the helicopter is flying at high speed, HSI (High-Speed Impulsive noise) is generated in a range from 60 to 300 Hz.